This invention generally relates to power supply and data communication systems for downhole tools or instruments. More particularly, this invention relates to a data communication system for a downhole instrument over a power cable.
Various communication systems exist for downhole instruments such as, but not limited to, electric submersible pump (“ESP”) gauges and surface controllers.
An ESP system includes a downhole motor and pump assembly, a surface- located control unit, and one or more downhole gauges or instruments. A three-phase AC power supply located at the surface provides an AC power signal over a three-conductor power cable to the downhole motor and pump assembly. Depending on the motor size and length of the power cable, the operating voltage of the motor can be very large. The three-phase AC power signal is coupled to the motor by a balanced inductor network having a neutral, ungrounded node. This node is referred to as the wye point of the motor or the downhole wye point.
The downhole instrument associated with the ESP measures physical parameters of the wellbore such as temperature and pressure. The telemetry data that represents those physical parameters must be communicated to the control unit and various schemes for doing so have been implemented. Because the instrument and its control circuitry include sensitive electronic components, they must be protected from high voltage events such as those that occur during a ground fault. Most of these ESP systems use large inductive isolation chokes—which have the disadvantage to limit the data transfer rate —but also make use of direct current power supplies, which can cause operations to stop in case of a ground fault on the power cable. For example, where DC power is tapped from the wye point of the motor, a ground fault can lead to higher than desired power levels at the wye point, thereby jeopardizing the instrument's sensitive electronic components.
Some systems couple the downhole instrument to the motor wye point and provide a surface-located AC power supply to generate power at a higher frequency than the motor power supply frequency. These systems require high voltage capacitors located between the downhole instrument and the motor wye point (see e.g. U.S. Pat. No. 7,982,633 B2 to Booker et al. and U.S. Pat. No. 8,138,622 B2 to Layton et al.). The capacitors are large in size, expensive, and have uncertain reliability.
Some systems protect the downhole electronics from high AC voltage using semiconductor devices by adding circuitry below the wye point which makes use of a diode (and associated voltage clamp) that conducts during positive polarity voltage and a silicon-controlled rectifier (and associated resistor) that conducts during application of a negative polarity voltage to the instrument (see e.g. U.S. Pat. No. 8,149,552 B1 to Cordill). However, those systems still require the use of a large inductive choke (e.g., in a range of about 80 H or greater), and the semiconductor devices only function to keep the choke current balanced during ground fault conditions (see also e.g. U.S. Pat. No. 6,176,308 B1 to Pearson).
A need exists for a system which eliminates the need for large inductive isolation chokes and high voltage capacitors while still protecting the downhole instrument and allowing the downhole instrument to operate and communicate during ground fault conditions.